Development of avalanche photodetectors for long wavelength (1.0 .mu.m.ltoreq..lambda..ltoreq.1.65 .mu.m) high bit rate communication systems has been impeded by the presence of an excessive amount of dark current. Excessive dark current, especially the tunneling component thereof, is a significant source of detector noise. The tunneling component of the dark current is enhanced by the high electric fields required to produce avalanche gain. For an explanation of the tunneling component of dark current in long wavelength avalanche photodiodes, see S. R. Forrest et al., Appl. Phys. Lett., 37(3), pp. 322-5 (1980).
In an effort to substantially eliminate the tunneling component of the dark current, the structure of long wavelength avalanche photodetectors employing a ternary or quaternary Group III-V semiconductor compound has been modified to include separate regions for multiplication and absorption. These separate regions accommodate a high avalanche field and a low interface field to reduce the tunneling current. One such avalanche photodiode employing a multiplication region of InP and a separate absorption region of InGaAsP has been demonstrated by K. Nishida et al. in Appl. Phys. Lett., 35(3), pp. 251-3 (1979). Similarly, O. K. Kim et al. have shown an avalanche photodetector structure including a multiplication region of InP and a separate absorption region of InGaAs in Appl. Phys. Lett., 39(5), pp. 402-4 (1981).
While separate absorption and multiplication region avalanche photodetectors have improved dark current characteristics, these photodetectors exhibit a frequency response which is slower than desired. This response is limited by a slow transient response which is attributed to an accumulation of charge at the valence band discontinuity between the multiplication and absorption regions, i.e., at the heterojunction interface.
To reduce the charge accumulation at the heterojunction interface, S. Forrest et al. in Appl. Phys. Lett., 41(1), pp. 95-8 (1982) have suggested compositional grading of the heterojunction interface over a distance of 600 to 1000 angstroms thereby reducing the valence band discontinuity.
A subset of the above-described technique for reducing the charge accumulation is growth of an intermediate bandgap, grading layer between the wide bandgap, multiplication region and the narrow bandgap, absorbing region. Although such a structure has valence band discontinuities, these discontinuities are relatively small. Y Matsushima et al. in IEEE Electron Device Letters, Vol. EDL-2, No. 7, pp. 179-181 (1981) show the use of three quaternary layers (InGaAsP) to form the grading region. Each quaternary layer increases in bandgap from the layer adjacent to the absorbing region to the layer adjacent to the multiplication region. The quaternary layer adjacent to the absorption region has a bandgap of 0.8 eV (.lambda..sub.g =1.55 .mu.m), the middle quaternary layer has a bandgap of 0.95 eV (.lambda..sub.g =1.3 .mu.m); and the quaternary layer adjacent to the multiplication region has a bandgap of 1.08 eV (.lambda..sub.g =1.15 .mu.m). Matsushima et al. in Elect. Lett., Vol. 18, No. 22, pp. 945-6 (1982) show the use of two quaternary layers in proper bandgap sequence to form the grading region. In the latter avalanche photodetector, the quaternary layer adjacent to the absorption region has a bandgap of 0.8 eV (.lambda..sub.g =1.55 .mu.m) whereas the quaternary layer adjacent to the multiplication region has a bandgap of 0.95 eV (.lambda..sub.g =1.3 .mu.m).
While Matsushima et al. claim an improved pulse response, via reduced charge accumulation at the heterojunction interface, for the photodetector including the two quaternary layers as the grading layer, it is evident that the device is incapable of both operating with high quantum efficiency and providing high speed response (&gt;200 MHz) because the device is unable to achieve both high light absorption and rapid carrier transport in the absorption region simultaneously.
From the descriptions given above, it is clear that avalanche photodetectors for high speed communication should have the properties of low dark current, high quantum efficiency, and rapid response time. In order to achieve these properties, device designers must provide a structure which accommodates a high avalanche field, a low interface field, and a depleted absorption region.